Choice "D" is correct. When there are few good substitutes for a supplier's product, the supplier has market power (think of a monopoly). As a result, the supplier is better able to control buyers and act as a price setter rather than a price taker.
Choice "b" is incorrect. When supplier's products are not differentiated, buyers will be indifferent about which supplier they purchase from. In other words, if firms sell identical products (think of perfect competition) the product of one firm is a perfect substitute for the product of another firm. In this case, firms are price takers, not price setters.
Choice "a" is incorrect. When there are a large number of firms, no one firm has much market power. This is the case of either perfect competition (if all firms sell identical products) or monopolistic competition (if all firms sell slightly differentiated products).
Choice "c" is incorrect. If the purchasing industry is an important customer of the supplier, the purchasing industry (i.e., the buyer) will have some market power. This will diminish the ability of the supplier to influence or control the buyer.